1. Field of the Invention
The present invention relates to a pixel structure of a solid-state image sensor, more particularly to an improvement of pixels that constitute a solid-state image sensor that can be used as a time-of-flight type distance sensor, which measures time-of-flight by receiving reflection light of light irradiated to an object with the use of time-of-flight measurement method (TOF: Time of flight) and measures a distance to the object based on the time-of-flight, or an image sensor that obtains a three-dimensional image of the object. Particularly the invention relates to a pixel structure of a solid-state image sensor, which can be used as a pixel of a solid-state image sensor employing a charge sorting method that can be used under unknown background light illumination, such as a solid-state image sensor that is equipped with a plurality of charge-storage sections, discriminates photoelectrons generated by incoming light on the timing of incoming light and store them in the plurality of charge-storage sections in a sorting manner.
2. Description of the Related Art
Generally, there is known a so-called time-of-flight measurement method (TOF: Time of flight) in which intensity-modulated illumination light by pulse or high frequency is irradiated on an object, time-of-flight where reflection light from the object reaches an image sensor is measured to calculate distance.
As a solid-state image sensor that can be utilized as an image sensor used in such a time-of-flight measurement method, there is a sensor employing a charge sorting method, which is equipped with a photoelectric conversion section and a plurality of charge-storage sections, for example, discriminates electrons generated in photoelectric conversion section by incoming light on the timing of the incoming light, and stores the discriminated electrons in the plurality of charge-storage sections in a sorting manner.
Meanwhile, according to the solid-state image sensor employing the charge sorting method, a processing of distributing electrons generated in the photoelectric conversion section to a plurality of the charge-storage sections corresponding to the incoming timing of light was conventionally realized by applying a high frequency pulse voltage to gate electrodes installed directly above the oxide film of semiconductor surface.
However, due to multiple reasons such as variation of impurity concentration to be doped in a semiconductor substrate in the vicinity of the gate electrodes, physical shape instability of the gate electrodes, and damage or thickness variation of the oxide film, even if each gate electrode is equipped with the same structure and the same voltage is applied to each gate electrode, there is a fear that passages (channels) of electrons formed on a semiconductor substrate directly under each gate electrode generally do not become completely the same, and it causes a possibility that channels formed directly under the gate electrodes are formed differently in between the gates to cause variation.
When such channels formed directly under each gate electrodes cause variation, electrons that should be distributed in a plurality of charge-storage sections under the same condition are distributed unevenly in a charge-storage section adjacent to a gate electrode having a low threshold value, and a problem that electrons generated in photoelectric conversion section cannot be rightly distributed to the charge-storage sections was pointed out.
Conventionally, to solve the problem regarding the distribution of electrons generated in the photoelectric conversion section, as disclosed in Japanese Patent Laid-open No. 2005-235893 presented as Patent Document 1, for example, a method in which a photo gate structure is employed in the photoelectric conversion section, by which charge residue is removed to generate a potential difference exceeding variation of a threshold value of gate, is proposed.
However, the fact that the photoelectric conversion efficiency of a photo gate is low to light having a long wavelength is known, and raised a new problem that the solid-state image sensor with the photoelectric conversion section employing the photo gate structure had a large obstacle in applying to various applications.
[Patent Document 1]
Japanese Patent Laid-open No. 2005-235893